The present invention relates generally to gas fuel admission systems for use with internal combustion engines. In particular, the invention relates to a gas fuel admission system with a gas admission valve which meters mass flow of gas to internal combustion engines.
Gas fuel systems are used for a variety of internal combustion engines, such as reciprocating or turbine engines used on vehicles and in industrial settings. These engines may utilize fuel systems that provide natural gas (predominantly methane) from liquid natural gas (LNG) or compressed natural gas (CNG) sources, or other gaseous fuels such as propane or hydrogen.
For a gas flowing through an orifice or valve at a temperature (T.sub.1), there is a critical pressure ratio (P.sub.CR) of the discharge pressure (P.sub.2) to the supply pressure (P.sub.1) for which flow through the orifice is unaffected by P.sub.2 whenever P.sub.2 &lt;(P.sub.CR .times.P.sub.1). This is a well known compressible flow phenomena referred to as sonic or choked flow. The critical pressure ratio P.sub.CR is not the same for all gases, for instance it is 0.53 for air, 0.54 for natural gas, and 0.58 for propane. In essence, all gas fuel admission systems must utilize sonic flow, subsonic (non-choked) flow or a combination of both flow regimes. Because of the relatively high air supply pressures used with the intake systems of modern gas internal combustion engines, a vastly simplified system could be achieved if the fuel admission valve could always operate in the sonic regime.
As a good example within this field, gas fuel admission systems have been used for vehicular and industrial LNG and CNG engines. The fuel systems on these engines are moving from traditional natural gas carburetors to precise mass flow control fuel injection systems with electronic engine controllers. The move is for many of the same reasons that automobile gasoline engines moved to fuel injection systems, namely, to reduce emissions, improve fuel economy, improve driveability, improve cold starting and enhance performance.
Large engines used in on-highway (truck and bus) applications generally utilize turbocharging systems to enhance engine performance. The fuel injection systems feed the gas into the engine downstream of the turbocharger air pressurizing function. Therefore, with present lean-burn engines, fuel admission valve discharge pressures P.sub.2 can range from ambient to in excess of 40 psia at full turbo boost. For LNG systems, gas fuel supply pressures P.sub.1 can be as low as 55 psia. Assuming the fuel is natural gas, with a critical pressure ratio P.sub.CR =0.54 and a supply pressure P.sub.1 =55 psia, a fuel system on a turbocharged engine using present fuel admission valve technology could only operate in the sonic flow regime up to a maximum turbocharged intake pressure of approximately 30 psia. The fuel supply system would operate in subsonic flow for turbocharger (and resultant fuel system discharge) pressures P.sub.2 above 30 psia.
Presently, there are essentially three fundamental methods of accomplishing precise mass flow control of a gas. The first uses a proportional valve/actuator, mass flow feedback sensor, and microprocessor controller in a closed operational loop. With direct measurement by a mass flow sensor, the valve is driven up or down until the indicated mass flow equals the commanded mass flow. However, this method has disadvantages. Economical mass flow sensors have not yet achieved a reputation for excellent reliability and long term ability to hold calibration. In addition, overall transient response is limited by the mass flow feedback sensor lag as well as the lag in the closed loop processing to position the valve. Finally, the available automotive grade mass flow sensors are intended for use in measuring the mass flow of air. Consequently, when used for natural gas at much lower flow rates, they do not operate at their designed optimum conditions. Examples of systems using this approach can be seen in U.S. Pat. Nos. 4,838,295 to Smith et al. and No. 5,146,941 to Statler.
The second method uses a proportional valve/actuator, gas supply pressure (P.sub.1) sensor, gas valve discharge pressure (P.sub.2) sensor, gas supply temperature (T.sub.1) sensor, precise valve position sensor, and microprocessor controller in a closed operational loop. Stored in memory is a map of effective valve area (C.sub.v) versus valve position. The algorithms for subsonic and sonic flow are different. Therefore, the flow calculation routine in the controller must first determine whether subsonic or sonic flow exists, and then must select and apply the appropriate algorithms to drive the valve up or down until the calculated mass flow equals the commanded mass flow. Although this method uses sensors that are more simple and well-proven than the mass flow sensors of the first method, this method has the disadvantage of using many sensors. This presents increased reliability concerns, and because errors are cumulative, to achieve excellent mass flow control accuracy, all sensors must be exceptionally accurate and stable over the long term. Moreover, the use of multiple sensors and subsonic as well as sonic flow regimes requires intensive calculation processing power. Transient response of the system is thus limited by the sensor lag times and the more intensive calculations required. Examples of systems using this approach can be seen in U.S. Pat. No. 5,388,607 to Ramaker et al. and U.S. Pat. No. 5,488,969 to King et al.
The third method uses a proportional valve/actuator in which the valve always runs in the sonic flow regime using a gas supply pressure (P.sub.1) sensor and a gas supply temperature (T.sub.1) sensor for density correction. This method has some advantages for use with gases (compressible flow) in that a minimum number of sensors are required, the correction for P.sub.1 and T.sub.1 variations are relatively simple, the calculations for valve positioning are not as intensive, and if it is known that gas valve discharge pressure P.sub.2 will always be less than P.sub.CR .times.P.sub.1, then no P.sub.2 sensor is required.
However, this third method does have some disadvantages. Given the critical pressure ratios P.sub.CR for gases, as an alternative to having to add a P.sub.2 sensor and operate at subsonic levels with additional algorithms, for the fuel system to operate only in sonic flow it generally must have a higher supply pressure than other methods. In fact, the necessary supply pressure would likely exceed the tank pressure for LNG applications. Moreover, higher supply pressure also limits the driving range of vehicles having CNG sources.
Systems operating by the third method have used two different techniques. First, a multitude of pulse width modulated (PWM) solenoid valves may be employed. The solenoid valves must open and close rapidly to obtain variable sonic flow. The valves tend to have durability problems and packaging can be problematic because engines capable of generating as much as 300 horsepower may require as many as eight solenoid valves in one common housing.
In a second configuration that utilizes the third method, a series of choked flow valves, each one flowing at twice the fuel capacity of its predecessor, are arranged for activation of however many of the valves are necessary to achieve the desired flow. These valves are not PWM but the system requires a multitude of valves, invoking similar packaging problems and actuation complexities.